The Casimir effect is a nonlinear attractive force between conducting plates that arises from a quantized vacuum field around the plates. Such a force can be induced by virtual photons that fill the vacuum field, and the force varies based on the separation between the conducting surfaces of the plates. Certain photon modes are forbidden from the area of the separation between the plates, as these modes have wavelengths that are too large to fit within the separation. Due to this phenomenon, the energy density is lower between the plates than it is outside the plates, and a pressure is formed that pushes the plates together.